1. Field of the Invention
The present invention relates to a controller for an axial-gap motor.
2. Description of the Related Art
Hitherto, there has been known an axial-gap motor equipped with a rotor having permanent magnets, two stators provided, one on each side of the rotor, in the axial direction of the rotor, and armature windings wrapped around each of the stators, as disclosed in, for example, Japanese Unexamined Patent Application Publication No. H10-271784 (hereinafter referred to as patent document 1) and Japanese Unexamined Patent Application Publication No. 2001-136721 (hereinafter referred to as patent document 2). Such an axial-gap motor is capable of generating a relatively high output torque while reducing the length of the rotor of the motor in the axial direction.
In an axial-gap motor, an attraction force is generated in the axial direction or the thrust direction of the rotor between the rotor and each of the stators due to magnetic fluxes produced by the permanent magnets provided in the rotor. In this case, if the position of the rotor in the axial direction is a position where the magnitudes of the attraction forces between the rotor and the individual stators are the same (hereinafter referred to as “the neutral position”), then the attraction forces usually cancel each other, so that the forces or the thrusting forces in the axial direction do not act on the rotor. Normally, in an axial-gap motor, one stator and the armature windings wrapped around the one stator and the other stator and the armature windings wrapped around the other stator share the same construction. Therefore, the aforesaid neutral position is usually a position at which the interval or the gap between the rotor and one stator is the same as that between the rotor and the other stator.
However, there is a case where the position of the rotor in the axial direction deviates from the neutral position because of an assembly error of the motor or a dimensional error of individual constituent elements, or the like. In such a case, the rotor will be subjected to a thrust force toward one of the stators. Especially when the rotor is provided with permanent magnets in a Halbach array, the rotor will be subjected to a relatively large thrust force even if the rotor is slightly dislocated from the neutral position, because the density of the magnetic fluxes generated from the rotor is high.
If the motor is run, rotating the rotor while the rotor is being subjected to a thrust force as described above, the bearing that supports the rotor or an output shaft connected thereto onto the housing of the motor will be subjected to the thrust force between itself and the housing. This inconveniently leads to wear or the like on the bearing with resultant deteriorated durability of the bearing or mechanical resonance which causes vibration in the thrust direction.